In the related art, information is transmitted between an electronic device and a peripheral device via a cable such as a universal serial bus (USB) cable. In addition, in recent years, devices are often connected in a cable-less manner by employing wireless transmission technologies. So as to maintain the portability provided by the cable-less technologies, there are also devices that transmit power wirelessly.
For example, in a cellular phone or the like, a technology capable of performing power transmission in a non-contact, wireless manner has been put to practical use. In such a technology, a power supply circuit and a power receiving circuit compatible with a non-contact power supply method such as an electromagnetic induction method or a magnetic field resonance method are used. By installing such a power supply circuit and a power receiving circuit in a power supply side device and a power receiving side device, respectively, it is possible to realize a non-contact power supply system that performs power transmission from the power supply side device to the power receiving side device in a cable-less manner. When an electric load of the power receiving side device will be large, a plurality of power receiving circuits can be connected in series with the electric load and power is supplied to each power receiving circuit from a plurality of power supply circuits provided in the power supply side device. In such a configuration, there is a possibility that an unbalance in the operating load between the power supply circuits (or the power receiving circuits) occurs. When the unbalance of the operating load occurs, there is a possibility that the charging/power supply operation becomes unstable, thus it is desired to eliminate the unbalance.
In the related art, to supply electric power to an electric load requiring a large current, there is a power conversion system in which a plurality of the power conversion devices are connected in series to a common voltage source and power is supplied from the plurality of the power conversion devices to the electric power load. In addition, to solve the unbalance of the operation load generated in each power conversion device, a technology has been proposed in which the output power of the power conversion device is set based on the deviation between an output voltage of the power conversion device and a target voltage and the output voltage is reduced when an input voltage of the power conversion device falls below a reference value set based on the power supply voltage.
However, the above-described related art is not for a non-contact power supply system, and it may be difficult to implement it in a non-contact power supply system. In addition, in the above-described technology in the related art, since the reference value varies according to the power supply voltage, there is a possibility that the configuration necessary to provide operational control becomes complicated.